SHAPING PROCESSES FOR PLASTICS Product Design Considerations ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: General Strength and stiffness Plastics are not as strong or stiff as metals Avoid applications where high stresses will be encountered Creep resistance is also a limitation Strength‑to‑weight ratios for some plastics are competitive with metals in certain applications ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: General Impact Resistance Capacity of plastics to absorb impact is generally good; plastics compare favorably with most metals Service temperatures Limited relative to metals and ceramics Thermal expansion Dimensional changes due to temperature changes much more significant than for metals ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: General Many plastics are subject to degradation from sunlight and other forms of radiation Some plastics degrade in oxygen and ozone atmospheres Plastics are soluble in many common solvents Plastics are resistant to conventional corrosion mechanisms that afflict many metals ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Extrusion Wall thickness Uniform wall thickness is desirable in an extruded cross section Variations in wall thickness result in non-uniform plastic flow and uneven cooling which tend to warp extrudate ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Extrusion Hollow sections Hollow sections complicate die design and plastic flow Desirable to use extruded cross‑sections that are not hollow yet satisfy functional requirements ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Extrusion Corners Sharp corners, inside and outside, should be avoided in extruded cross sections They result in uneven flow during processing and stress concentrations in the final product ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Analysis of Blow molding   ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Problem A blow molding operation produces a 6.25in diameter bottle from a parison that is extrude in a die whose outside diameter is 1.25in and inside diameter is 1 in. The observed diameter swell ratio is 1.24. what is the maximum air pressure is maximum tensile stress is 1000lb/in2 ? ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Economic production quantities Each part requires a unique mold, and the mold for any molding process can be costly, particularly for injection molding Minimum production quantities for injection molding are usually around 10,000 pieces For compression molding, minimum quantities are 1000 parts, due to simpler mold designs Transfer molding lies between injection molding and compression molding ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Part complexity An advantage of plastic molding is that it allows multiple functional features to be combined into one part Although more complex part geometries mean more costly molds, it may nevertheless be economical to design a complex molding if the alternative involves many individual components that must be assembled ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Wall thickness Thick cross sections are wasteful of material, more likely to cause warping due to shrinkage, and take longer to harden Reinforcing ribs Achieves increased stiffness without excessive wall thickness Ribs should be made thinner than the walls they reinforce to minimize sink marks on outside wall ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Corner radii and fillets Sharp corners, both external and internal, are undesirable in molded parts They interrupt smooth flow of the melt, tend to create surface defects, and cause stress concentrations in the part Holes Holes are quite feasible in plastic moldings, but they complicate mold design and part removal ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Draft A molded part should be designed with a draft on its sides to facilitate removal from mold Especially important on inside wall of a cup‑shaped part because plastic contracts against positive mold shape Recommended draft: For thermosets, ~ 1/2 to 1 For thermoplastics, ~ 1/8 to 1/2 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Product Design Guidelines: Moldings Tolerances Although shrinkage is predictable under closely controlled conditions, generous tolerances are desirable for injection moldings because of Variations in process parameters that affect shrinkage Diversity of part geometries encountered ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e